1. Field of the Invention
The present invention relates to a PWM driving apparatus which drives two loads connected in parallel to a power source by a PWM (Pulse Width Modulation) control.
2. Description of the Prior Art
Heretofore, there is described, for example in JP-A 2002-43910, a load controlling device which drives a plurality of loads connected in parallel to a power source. Such a load controlling device is of a pulse width modulation (PWM) control system for simultaneously driving first and second loads (for example, headlights) connected to the power source (for example, in-vehicle battery). In addition, this load controlling device includes first and second switching elements for applying power to the first and second loads respectively, and a controlling circuit which outputs first PWM signals to allow the first switching element to be turned ON and OFF upon receipt of signals for commanding driving and second PWM signals to allow the second switching element to be turned ON and OFF upon receipt of the signals for commanding driving, while the phase of the first PWM signals is staggered from that of the second PWM signals.
According to this structure, since the phases of the first and second PWM signals are staggered from each other, load currents flowing to the first and second loads will not be increased or decreased at the same timing, and the rate of variation in the entire load current becomes less as compared with a case that the two loads are driven with same phases. As a result, current as flowing in wiring from the power source to each load will not vary significantly, thus generation of an electromagnetic noise can be suppressed. Furthermore, an amount of variation in the entire load current can be made less by matching the rising timing of first PWM signals with the falling timing of the second PWM signals.
Also, there is described, for example in JP-A 2002-315392, a motor fan device for being mounted in a vehicle which is configured to reduce conduction noise generated due to a current ripple of an output current of the power source by driving two motor fans in such a manner that the current ripple of the output current of the power source becomes less. In this motor fan device, separate switch elements are prepared for the two motor fans respectively, and those respective switch elements are turned ON or OFF under the PWM control, while the ON-OFF operation is staggered between the switch elements by a half cycle. Thereby, driving currents are distributed from the power source to the respective motor fans, while being staggered from each other by a half cycle.
In the above mentioned devices, it is possible to suppress the generation of the electromagnetic noise by staggering the phases of the two PWM signals and thus shifting the timing of the power application between the respective loads. However, an amount of generation of the electromagnetic noise fluctuates corresponding to the duty of the PWM signals in a case that a phase difference between the two PWM signals is set to be constant. For example, when the phase difference between the two PWM signals is set to be 180 degrees (staggered by a half cycle), the amount of generation of the electromagnetic noise is less when the duty of the PWM signals is 50%, and the amount of generation of the electromagnetic noise increases when the duty of the PWM signals is not 50% as compared with the case when the duty is 50%. Accordingly, when an amount of electric power to be supplied to the load is varied (more specifically, when the duty of the PWM signals is varied), the amount of generation of the electromagnetic noise fluctuates, thus there is a case that the electromagnetic noise may be heard from a car-mounted radio as a disturbing noise.